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Hominid speciation, was h
J. Moore (j#d#.moore@canrem.com)
Tue, 18 Jul 95 11:59:00 -0500
Rl> I always have trouble with the argument that Tattersall and
Rl> others present based on degree of morphological difference. There are
Rl> populations such as sibling species that appear morphologically
Rl> identical and there morphologies so wildly different, i.e., great danes
Rl> and Dauschunds, being the same species. In these cases we can test
Rl> whether or not the populations are reproductively isolated, but with the
Rl> fossil record we are in no such position.
Rl> Ralph Holloway.
This gets into my position on australopithecines, and a
conversation I had with someone who certainly disagreed with me.
My feeling is that a major reason for australopithecine separation
was their apparently wildly different diets. Not so much in terms
of veggies vs. meat, in fact probably not at all; they probably,
almost certainly, both ate mostly veggies and grabbed whatever meat
they could, just as chimps do, just as humans who gather and hunt
do (when they're in climates that allow it [ie. not wintery] and
of course the humans are much more effective at getting meat
regularly). The diff would be that robust australopithecines
could and apparently did eat much rougher, tougher stuff -- hence
the jaws and jaw muscles. This likely put the two in different
areas, or in the same areas at different times.
I think this changed with the advent of sharpened stone tools.
(It should be noted that there is no evidence that supports the
common idea that habilis *must* have invented these handy tools;
and in fact there is evidence that robust australopithecines used
tools [finger bones, as noted by Susman].) This would allow for
both increased meat-eating (or marrow-eating, ala Binford) by
pirating from carnivores (I dislike the term scavenging, as it
implies eating long-dead carcasses, which seems unlikely) and
utilization of different plant foods due to easier and innovative
processing. Both these changes, and probably especially
meat-eating (because available meat is both rarer than plants and
more localised when dead ;-) would tend to bring these separate
populations into contact.
Contact between groups means sex. There's other things that go
on, including fighting and threats, but among humans we always
see historically, despite grave strictures against it, sex between
"others". I think we saw it then, and see it still, in the form
of fossils. When you mix groups you get variation; this was long
thought to be something seen only in plants to any degree, but is
apparently being recognised more as something seen in animals as
well. While reading *The Beak of the Finch*, I found that those
researchers had found that hybridization was an important source
of variation which leads to opportunities for speciation. This
was also pointed out among fruit flies by Lewontin and Birch in
1966. It's not just a plant thing. Variation gives source
material for natural (and sexual) selection, and hybridization
results in variation. Significantly, I think, we see a lot of
hominid variation soon after the advent of sharpened stone tools,
in the form of a highly variable "species", *Homo habilis*.
Several objections can be made to this, but the most common is
simply that "they were separate species, so they couldn't
interbreed". You can hear this, as I did, from someone who
also volunteers that some undeniably interbreeding extant species
are morphologically as dissimilar as the two varieties of
australopithecines, and who also volunteers that some undeniably
separate extant species cannot be told apart by examining, say,
their skulls. In spite of knowing that these examples exist, the
same person can (and did) repeat that we "know" that the two
varieties of australopithecines *could not* interbreed, because
they are two separate species. There are two answers to this.
One is that in fact we *don't know* whether or not they *could*
interbreed, as we can't tell this from fossils (unless you accept
my view on the matter ;-). The second is that different species
can in fact interbreed, and do not, as is commonly thought,
necessarily result in infertile hybrids. This matter can drift
quite a bit so, altho it's interesting, I won't get into here
unless someone feels we really need to get into the subject.
Suffice it to say that there is generally reduced fertility, and
that male hybrids are generally much less often fertile than
females (and that I've seen an interesting but possibly
coincidental similarity recorded in the ratios of fertile males
and fertile females in both fruit fly-crosses and in
buffalo-cattle crosses [!Law of Nature or coincidence? Who knows.]).
What I'm suggesting is that if the variation produce some really
useful beings, the reduced fertility may not be enough to make
them meld back into one population or the other. If, at a time
when a lot of innovation was happening already, with new food
sources being made available due to new technology, a bigger brain
might just be a really great thing to have. (Contrary to pop
thought, bigger and presumably better brains were not always
necessarily a big advantage during hominid evolution -- if you
don't need more smarts to get along, it conveys no advantage.)
One thing, perhaps the most noticeable and talked about thing, we
see in the highly variable habilines is bigger brains. At such a
time of innovation, more smarts *would* be useful, and therefore
might tend to proliferate as they convey advantage to their
bearers and their close relations (Note: my use of the word relations
instead of relatives or kin is deliberate; I mean family and
friends, members of one's group).
This view is not the normal way of looking at speciation in human
evolution, but I think it has merit, and that the possibility of
its being a correct view has been obscured by the problems of
attempting to differentiate species by the unfortunately necessary
but restrictive means of examining only fossils remains, and by
the lack of realization of the role of hybridization in producing
variation (as source material for the action of selection) in
animals as well as in plants.
Jim Moore (j#d#.moore@canrem.com)
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